Izuku herself is obviously a very popular character in this type of fanfiction. Since the whole story revolves around her transformation into a girl. Her friends like Ochaco Uraraka also tend to be popular as they often play important roles in Izuku's new life as a girl. Their relationship might change and develop in interesting ways.
Well, in some fanfictions of this kind, Izuku as a girl might discover new aspects of her Quirk. Since the physical change could potentially affect how she uses her power. Also, there could be a plotline where she has to deal with romantic interests in a new way. As a girl, she might attract different kinds of attention from her male classmates, and she has to navigate through those new feelings and situations.
Well, it could start with Izuku waking up as a girl one morning with no explanation. He (now she) tries to keep it a secret at first but then realizes it's not possible. The story could focus on how Izuku's relationships change. For example, Bakugo might initially tease her more harshly but then come to respect her in a different way. And Midoriya could find that she has a different approach to using her quirk as a girl, maybe being more strategic or having different physical capabilities that she has to learn to master.
Perhaps a jealous classmate who has always been in Izuku's shadow. Now that Izuku is a girl, this classmate sees an opportunity to undermine her. They spread rumors about how Izuku's transformation is some sort of cheat or that she's not the same reliable person she was as a boy. This classmate's actions and attempts to sabotage Izuku would make them the main antagonist.
Well, one possible plot could be that his friends have to take care of him. They might struggle at first, not knowing how to handle a baby Izuku. For example, Bakugo could be really frustrated but gradually learn to be gentle. Ochaco might be really good at soothing baby Izuku when he cries.
Well, obviously Izuku is one of the main characters, even when he's turned into a cat. His friends like Bakugo and Ochaco are also likely to be main characters. They would be the ones trying to figure out what happened to Izuku and how to turn him back. And maybe All Might could also be a main character, if he gets involved in the whole situation to help his young protege.
Definitely his friends from U.A. High. Bakugo would be an interesting character in this scenario. He might initially be annoyed but then develop a soft spot for baby Izuku. Ochaco would also be there, using her quirk in creative ways to take care of him, like floating toys to keep him entertained.
A more emotional plot could be Izuku as a cat being found by a villain who initially wants to use him against the heroes. But over time, the villain starts to develop a strange bond with the cat - Izuku. This then makes the villain question his own motives and actions.
Another aspect is the character development. He has to learn to control this new form, which means dealing with new instincts and emotions. It's like he has a whole new side of himself to explore and understand. For example, in some fanfictions, his dragon form makes him more confident in some ways but also more reckless in others. This internal struggle is really engaging to read about. Moreover, the relationships around him change too. His friends might be in awe or a bit scared at first, and how they all adapt to this new Izuku is a big part of the story.
He might do it for a bet. Just like some friends might bet on who can pull off a crazy stunt, his friends could have bet him that he can't dress like a girl and act the part for a certain period.